The present invention relates to wide-angle zoom lens systems. More particularly, the present invention relates to a wide-angle zoom lens system having a short optical total length not only at the wide-angle end but also at the telephoto end and a reduced telephoto ratio despite the fact that it has a high zoom ratio.
There has heretofore been a general tendency for zoom lens systems with a relatively low zoom ratio to use a two-unit zoom lens structure and for zoom lens systems requiring a higher zoom ratio to use a three-unit zoom lens structure. There are many variations on the zoom type of these conventional zoom lens systems. However, development of such variations has resulted in an increase in the number of lens units. To minimize the number of lens units or the number of constituent lens elements, some of the variations adopt a method wherein the aperture ratio at the telephoto end is set at a relatively small value and an aspherical surface is used.
Meanwhile, a four-unit zoom lens system for a compact camera is disclosed in U.S. Pat. No. 4,822,152 by the present applicant. The zoom lens system of U.S. Pat. No. 4,822,152 has, in order from the object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power, and a fourth lens unit of negative refractive power. When the zoom lens system is zoomed from the wide-angle end to the telephoto end, each of the lens units moves toward the object side.
The feature of the above zoom lens system resides in that all the lens units are moved to share the load of attaining a high zoom ratio. When the zoom lens system is zoomed from the wide-angle end to the telephoto end, the second lens unit is moved toward the object side, thereby attaining a reduction in size at the wide-angle end.
There have heretofore been made many propositions that an aspherical surface is used to attain a compact lens system. Attempts have also been made to reduce the number of lens elements constituting one lens unit by increasing the aberration correcting power assigned to an aspherical surface. There have also been proposed a large number of zoom lens systems that use a radial type gradient index lens.
Many of the actual zoom lens systems for compact cameras have a mechanism that allows the lens system to be stored in the camera body irrespective of the type of zoom lens systems. That is, the lens system is moved into the camera body by a collapsible lens barrel mechanism that is provided in a space remaining when the lens position is at the wide-angle end, at which the overall length of the lens system is the shortest, thereby realizing a reduction in size of the camera when the lens system is in the collapsed storage position.
However, when photography is actually performed, the lens system is placed in the position for forming the original optical system. Therefore, in the case of a high-zoom ratio zoom lens system, the lens barrel becomes very long because of a large amount of lens movement for zooming to the telephoto end, and the center of gravity also shifts. Accordingly, the entire optical system tends to be decentered undesirably.
In view of the above-described problems, an object of the present invention is to provide a wide-angle zoom lens system having a short optical total length not only at the wide-angle end but also at the telephoto end and a reduced telephoto ratio despite the fact that it has a high zoom ratio, with a view to achieving a reduction in size of cameras.
To attain the above-described object, the present invention provides a wide-angle zoom lens system having, in order from the object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power, and a fourth lens unit of negative refractive power. When the zoom lens system is zoomed from the wide-angle side to the telephoto side, each of the lens units moves toward the object side such that the space between the first lens unit and the second lens unit increases, whereas the space between the third lens unit and the fourth lens unit decreases. The wide-angle zoom lens system satisfies the following conditions:
0.7 less than f1/fW less than 3.5xe2x80x83xe2x80x83(1)
0.15 less than |f4|/fW less than 0.7xe2x80x83xe2x80x83(2)
0.2 less than xcex94X4T/fT less than 0.65xe2x80x83xe2x80x83(3)
2.5 less than xcex24T/xcex24W less than 6.0xe2x80x83xe2x80x83(4)
where f1 is the focal length of the first lens unit; fW is the focal length at the wide-angle end of the wide-angle zoom lens system; f4 is the focal length of the fourth lens unit; fT is the focal length at the telephoto end of the wide-angle zoom lens system; xcex94X4T is the amount of movement of the fourth lens unit for zooming from the wide-angle end to the telephoto end; xcex24T is the lateral magnification at the telephoto end of the fourth lens unit; and xcex24W is the lateral magnification at the wide-angle end of the fourth lens unit.
In this case, it is desirable that the first lens unit include at least one positive lens and one doublet consisting of a positive lens and a negative lens, and the second lens unit include at least one negative lens, and further the third lens unit include at least one doublet consisting of a negative lens and a positive lens, and further the fourth lens unit include at least a negative lens and a positive lens.
It is desirable that any one or more of the first to fourth lens units have at least one aspherical surface.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.